Answers

Bonsoir et bienvenue a GoY! You need to collect ripe seeds and sow them immediately in the autumn, either in a seed bed outdoors, or in pots. They germinate better if you keep them cold! Avez vous compris, Alexandre? Je ne parle pas bien francais.

Any plants you get from the seed may not be the same as the parent plant. Mahonia x media is a cross between two different parent plants so your 'children' may revert to one of them.
I hope you can understand this!